Process of preparation of synthetic fire opal

ABSTRACT

A synthetic fire opal and a process for its preparation. Synthetic fire opals have similar physical and chemical properties as natural fire opals. Synthetic fire opals are colored, hard, and transparent. The process of preparation of synthetic fire opal includes; mixing TEOS with absolute ethanol (99.9%), distilled water, concentrated nitric acid, and an inorganic salt to form a clear sol, storing the clear sol to obtain a gel, drying the gel and sintering the dried gel to obtain a synthetic fire opal.

BACKGROUND

The present invention relates to the field of synthetic gemstones. Morespecifically, it relates to a method of preparation of a synthetic fireopal that has similar physical and chemical properties as a natural fireopal.

Natural opal is a gemstone made of hydrated amorphous silica (chemicalformula SiO₂.nH₂O). Based on visual appearance, natural opals can beclassified as precious opals, common opals, and fire opals. Preciousopals exhibit a ‘play-of-color’. Play-of-color is the phenomenon ofchanging spectral hues produced by the diffraction of white lightthrough a microstructure of orderly arrayed silica spheres. Common opalshave high opacity and do not exhibit the play-of-color. Fire opals aretransparent or translucent opals with an orange or red body color.

Natural fire opal is an opal that has been mined from the earth and maybe treated in a number of ways to enhance its qualities. Natural fireopals are rare and expensive. Also, in natural opals, there is a lot ofvariation in color from piece to piece. Natural fire opals have longbeen used as ornamental stones in jewelry. However, natural fire opalsare brittle and heat sensitive. Most natural fire opals also showcrazing. Crazing is the phenomenon of developing cracks in the opal withthe passage of time or when the fire opal is subjected to heating.

Simulants of natural opals exist in the market. These simulants are usedas alternatives for natural opals. Simulants are artificial stones thatresemble the natural opal, but their chemical composition and physicalproperties are not the same as natural opals. Synthetic opals have thesame chemical composition and physical properties as natural opals.Scientists have been able to produce synthetic opals in laboratoryconditions.

A paper titled ‘Some Aspects of Precious Opal Synthesis’, by S. V.Filin, A. I. Puzynin, and V. N. Samoilov, published in ‘AustralianGemmologist’, Vol. 21, No. 7, 2002, pp. 278-282, describes one suchmethod of preparation of synthetic opals. The paper describes thesynthesis of an opal by a multi-step process. In the first stage of theprocess, synthesis of monodisperse particles of silica of about 300 nmsize is carried out by the hydrolysis of tetraethyl ester oforthosilicic acid, Si(OC₂H₅)₄ in ethanol. In the second stage, the rawprecursor of opal is precipitated by spontaneous sedimentation. Thesedimentation time is about seven months. In the third stage, theprecursor opal is dried by a method of supercritical drying in anautoclave to obtain samples of any dimension within a time span of 10-12hours. This is followed by sintering in a furnace at temperatures of600° C., 800° C., and 1000° C. The total time involved in the synthesisof a synthetic opal is 10 months.

U.S. Pat. No. 4,703,020, titled ‘Process for the Production of Jewellingand Ornamental Material’, assigned to Kyocera Corporation, Kyoto, Japan,describes another such method of production of synthetic opal. Thispatent describes a process of production of a jewelling and ornamentalmaterial which produces a play-of-color similar to a natural opal. Themain raw material used is amorphous silica spheres with a particlediameter within the range of 0.15 to 0.4 microns. The mixture ofamorphous silica spheres and water is subjected to sedimentation forseveral weeks to several months. The resulting jelly-like precipitate isspontaneously dried, and then calcined to a temperature of about 800° C.to form a structure composed of regularly three-dimensionally arrangedamorphous silica spheres. Zirconium alkoxide, in the form of a solution,is impregnated in the three-dimensional structure. The entire structureis then calcined at a temperature of about 1150° C. for about 30 hoursto obtain a jewelling and ornamental material.

The above-mentioned methods of preparation of synthetic opals dependupon the three-dimensional arrangement of amorphous silica spheres toproduce play-of-color. Therefore, such methods may not be applied forthe preparation of synthetic fire opals without play-of-color. Inaddition, such methods may not lead to the production of synthetic fireopals in different colors. Another limitation is the time taken by eachof these processes. The entire process in the existing methods ofpreparation of synthetic opals takes about 7-10 months. Further, theyalso require production of amorphous silica spheres with particlediameter in the specified range, as the first step in the process ofpreparation of a synthetic opal.

It is therefore evident that there is a need for a process ofpreparation of synthetic fire opals. The process should producesynthetic fire opals with color uniformity. The process should be lesstime consuming and should lead to the preparation of synthetic fireopals in different colors. Further, the process should produce syntheticfire opals, not limited by the particle size of the raw material.

SUMMARY

It is an object of the invention to provide a process for thepreparation of a synthetic fire opal for use in jewelry and ornaments.

The disclosed method involves the following steps: Absolute ethanol(99.9%), concentrated nitric acid, distilled water, and an inorganicsalt are mixed in a closed container at room temperature and atmosphericpressure. Then, the mixture is vigorously stirred for 1 to 10 hours byusing a magnetic stirrer to obtain a homogeneous reaction mixture.Thereafter, Tetraethylorthosilicate (TEOS) is carefully added to thehomogeneous reaction mixture to obtain a clear sol. The clear sol isstored at room temperature for 5 to 10 days to obtain a wet gel. The wetgel is dried at temperatures ranging from 50° C. to 100° C. to obtain acolored and transparent material. Subsequently, the dried gel is furtherheat treated at temperatures ranging from 250° C. to 800° C. to obtain acolored, transparent, and hard synthetic fire opal.

The synthetic fire opal has the same physical and chemical properties asthat of a natural fire opal. The synthetic fire opal is characterized bya hardness of 5.5 to 6.5 on the Mho scale, at least two percent of watercontent, refractive index of 1.37 to 1.47, and specific gravity of 1.96to 2.20. Further, the synthetic fire opal may be of any color, forexample, orange, pink, red, green, yellow, blue and the like. Thesynthetic opal may also be colorless.

In accordance with one embodiment of the invention, the process ofpreparation of synthetic fire opal takes about 4 to 6 weeks, which issubstantially less in comparison to already known processes of syntheticopal production. Further, in the present invention, the process ofpreparation of synthetic fire opal is not affected by the particle sizeof the raw material. There is no requirement for uniform size particlesof silica in this method. The present invention leads to the preparationof synthetic fire opal with color uniformity. Further, the syntheticfire opal so prepared may be of any color, i.e., orange, pink, red,green, yellow, blue and the like. The synthetic fire opal may also becolorless. The synthetic fire opal is hard and transparent. Thesynthetic fire opal is more stable than natural fire opal and does notshow crazing. The synthetic fire opal can be used to make beads,cabochons, cut stones, or other shapes that can be used in various typesof jewelry and ornaments.

BRIEF DESCRIPTION OF THE DRAWINGS

Various embodiments of the invention will hereinafter be described inconjunction with the appended drawings provided to illustrate and not tolimit the invention, wherein like designations denote like elements, andin which:

FIG. 1 is a flowchart illustrating the process of preparation of asynthetic fire opal for use in jewelry and ornaments, in accordance withan embodiment of the present invention;

FIG. 2 is a flowchart illustrating the process of hydrolysis of TEOS toobtain a clear sol, in accordance with an embodiment of the presentinvention;

FIG. 3 is a flowchart illustrating the process of drying a wet gel toobtain a colored and transparent material, in accordance with anembodiment of the present invention; and

FIG. 4 is a flowchart illustrating the process of sintering the driedgel to obtain a synthetic fire opal, in accordance with an embodiment ofthe present invention.

DESCRIPTION OF EMBODIMENTS

For the sake of convenience, the terms used to describe variousembodiments are defined below. It should be understood that these areprovided to merely aid the understanding of the description, and thatdefinitions in no way limit the scope of the invention.

Hydrolysis: The chemical reaction between water and a compound, usuallya salt. The hydroxyl from the water combines with a cation from thecompound and the hydrogen ion from the water combines with as anion fromthe compound.

Sol: A colloid that has a continuous liquid phase in which a solid issuspended in a liquid.

Gel: A colloidal dispersion that shows some rigidity and will keep theshape of the container in which it has been placed.

Gelation: The formation of a solid material from the modification ordestabilization of a liquid. Gels have varying degrees of hardnessdepending on the solid concentration of the mass.

Drying: Removing of water (90-95%) or organic solvent from a material,by exposure to air, heating, or adding a chemical drying agent.

Sintering: The heating of a mass of fine particles (e.g., leadconcentrates) below the melting point, causing agglomeration to formlarger particles.

Synthetic fire opal: Synthetic fire opal is a material with essentiallythe same chemical composition and physical structure as natural opalsbut has been made in a laboratory or through an industrial process.

Synthetic fire opals are also referred as created fire opals.

The present invention describes a synthetic fire opal for use in jewelryand ornaments and a process for preparation thereof.

In accordance with one embodiment of the invention, the main rawmaterials used for the preparation of the synthetic fire opal are aprecursor for silicon, a catalyst, a solvent and at least one inorganicsalt. TEOS is used as a precursor for silicon. It will be apparent to aperson skilled in the art that tetramethylorthosilicate (TMOS),tetramethoxysilane (TMS), tetraethoxysilane (TES) and the like may alsobe used as precursors for silicon instead of TEOS. TEOS undergoeshydrolysis and condensation reaction to form soluble metal hydroxides,which ultimately leads to the formation of continuous (—Si—O—Si—)linkages in an inorganic network that spans throughout the solventmedium. Absolute ethanol of 99.9% purity is used as a solvent in theprocess. Distilled water is required for hydrolysis of TEOS in theprocess. Concentrated nitric acid is used as a catalyst in the process.The assay concentration of nitric acid is 68 to 72%. It will be apparentto a person skilled in the art that other catalysts like HCl, ammonia,and glacial acetic acid etc., may also be used as catalysts withoutdeviating from the scope of the present invention. Various inorganicsalts of copper, cobalt, nickel, and iron are used as dopants to obtainsynthetic fire opal of different colors. For example, salt of iron maybe used to obtain an orange synthetic fire opal; salt of cobalt may beused to obtain a pink synthetic fire opal; another salt of cobalt may beused to obtain a red synthetic fire opal; salt of nickel may be used toobtain a green synthetic fire opal; salt of iron may be used to obtain ayellow synthetic fire opal; and salt of copper may be used to obtain ablue synthetic fire opal. It will be apparent to a person skilled in theart that it is a cation in the inorganic salt that is responsible forthe color of synthetic fire opals. The inorganic salts in the presentinvention may have any anion, i.e., nitrate, chlorate, carbonate,sulphate, and the like.

FIG. 1 is a flowchart illustrating the process of preparation of asynthetic fire opal for use in jewelry and ornaments in accordance withan embodiment of the present invention. At step 102, hydrolysis of TEOSis carried out in a closed container at room temperature and atmosphericpressure to obtain a clear sol. This step is described in detail inconjunction with FIG. 2. At step 104, the clear sol obtained at step 102is stored at room temperature in a closed cupboard for a period of 5 to10 days and is transformed into wet gel. At step 106, the wet gelobtained at step 104 is dried at temperatures ranging from 50° C. to100° C. to obtain a colored and transparent material. In an embodimentof the present invention the colored and transparent material is a drygel. This step is described in detail in conjunction with FIG. 3. Atstep 108, the colored and transparent material obtained at step 106 issintered, i.e., further heat treated, at temperatures ranging from 250°C. to 800° C. to obtain a colored, hard, and transparent synthetic fireopal. This step is described in detail in conjunction with FIG. 4.

FIG. 2 is a flowchart illustrating the steps involved in mixing of rawmaterials to obtain a clear sol in accordance with an embodiment of thepresent invention. At step 202, absolute ethanol (99.9%), distilledwater, concentrated nitric acid, and at least one inorganic salt(selected in accordance with the desired final color as described above)are mixed in a closed container at room temperature and atmosphericpressure. At step 204, the mixture of step 202 is vigorously stirred for1 to 10 hours by using a magnetic stirrer to obtain a homogeneousreaction mixture. At step 206, TEOS is carefully added to thehomogeneous reaction mixture of step 204 to obtain a clear sol. Themixing and stirring, as described above, results in the hydrolysis ofTEOS.

The clear sol obtained after the hydrolysis of TEOS is stored at roomtemperature. In an embodiment of the present invention the clear sol isstored in a closed cupboard for a period of 5 to 10 days. After 5 to 10days, the clear sol is transformed into wet gel, which is used forfurther processing.

FIG. 3 is a flowchart illustrating the steps involved in drying a wetgel to obtain a colored and transparent material in accordance with anembodiment of the present invention. At step 302, the wet gel obtainedis dried in a laboratory furnace at temperatures ranging from 50° C. to100° C. At step 304, the temperature at which the drying is done ismaintained until a colored and transparent material is obtained. In anembodiment of the present invention the colored and transparent materialis a dry gel.

FIG. 4 is a flowchart illustrating the steps involved in sintering thedried gel to obtain a synthetic fire opal in accordance with anembodiment of the present invention. At step 402, the colored andtransparent material obtained as a result of drying the wet gel (asshown in FIG. 3) is further heat treated at temperatures ranging from250° C. to 800° C. At this step, the organic part of the colored andtransparent material gets removed. Also, at this step, most of the wateris removed from the colored and transparent material to prevent crazing.At step 404, the temperature at which the sintering is done ismaintained until a colored, transparent and hard synthetic fire opal isobtained. The colored, transparent and hard synthetic fire opal is usedin jewelry and ornaments.

In another embodiment of the invention, a synthetic fire opal withoutany water content is obtained. In the process described above, thecolored and transparent material obtained at step 304 is further heatsintered at temperatures beyond 1000° C. to obtain a synthetic fire opalwithout any water content.

In another embodiment of the invention, the wet gel obtained at step 104is subjected to supercritical drying in an autoclave to obtain asynthetic fire opal without any water content.

In another embodiment of the invention, a synthetic common opal isobtained. The common opal is translucent to opaque withoutplay-of-color. Further, the synthetic common opal so prepared may be ofany color i.e. pink, green, and blue and the like. The synthetic commonopal may also be colorless.

Exemplary Embodiment

In an exemplary embodiment of the invention, absolute ethanol (99.9%),distilled water, concentrated nitric acid and salt of iron are mixed ina closed container at room temperature and atmospheric pressure. Themixture is vigorously stirred for 1 to 10 hours using a magnetic stirrerto obtain a homogeneous reaction mixture. TEOS is carefully added to thehomogeneous reaction mixture to obtain a clear sol. The mixing andstirring, as described above, results in the hydrolysis of TEOS.

The clear sol obtained after the hydrolysis of TEOS is stored at roomtemperature in a closed cupboard for a period of 5 to 10 days. After 5to 10 days, the clear sol is transformed into wet gel, which is used forfurther processing.

The wet gel thus obtained is dried in a laboratory furnace attemperatures ranging from 50° C. to 100° C. to obtain an orangetransparent material.

The orange transparent material obtained as a result of drying the gelis sintered i.e., further heat treated at temperatures ranging from 250°C. to 800° C. During this process organic part of the orange transparentmaterial gets removed and the water content is also reduced to preventcrazing. The final product obtained is an orange, transparent and hardsynthetic fire opal for use in jewelry and ornaments.

The orange synthetic fire opal has the same chemical composition andphysical properties as a natural fire opal. The synthetic fire opal hasthe hardness of 5.5 to 6.5 on Mho's scale, water content of at least 2%,refractive index of 1.37 to 1.47 and specific gravity of 1.98 to 2.20 asthe natural fire opal. The process for the preparation of the orangesynthetic fire opal takes about 4 to 6 weeks. Further, the process forpreparation of the orange synthetic fire opal in the present inventionis not affected by the particle size of the raw material. The orangesynthetic fire opal is more stable and does not show crazing. The orangesynthetic fire opal can be used to make beads, cabochons, or cut stones,which can be used in various kinds of jewelry and ornaments.

Exemplary Colors

In another embodiment of the invention, a pink synthetic fire opal isobtained. In the process described above, a salt of cobalt is used inthe starting mixture to obtain a pink synthetic fire opal. Usage of asalt of cobalt may also lead to the production of a red synthetic fireopal.

In another embodiment of the invention, a salt of nickel is used in thestarting mixture to obtain a green synthetic fire opal.

In another embodiment of the invention, a blue synthetic fire opal isobtained. In the process described above, a salt of copper is used as aninorganic salt in the starting mixture to obtain a blue synthetic fireopal.

In another embodiment of the invention, a salt of iron is used as aninorganic salt in the starting mixture and the sintering of the driedgel is done in an oxidizing atmosphere to obtain an orange syntheticfire opal. Further, in another embodiment of the invention, a yellowsynthetic fire opal is obtained by adding an inorganic salt of iron inthe starting mixture and sintering the dried gel in nitrogen atmosphere.It will be apparent to a person skilled in the art that in the processdescribed above varying the atmospheric conditions at which thesintering is done may lead to the production of a different coloredsynthetic fire opal.

In another embodiment of the invention, a colorless synthetic fire opalis obtained. In the process described above, no inorganic salt is usedin the starting mixture to obtain a colorless synthetic fire opal.

The synthetic fire opal has the same chemical composition and physicalproperties as natural fire opal. The synthetic fire opal has thehardness of 5.5 to 6.5 on Mho's scale, water content of at least 2%,refractive index of 1.37 to 1.47, a body color of orange to red tone andspecific gravity of 1.98 to 2.20 as the natural fire opal. The syntheticfire opal prepared by the present invention can be obtained in differentcolors, for example, orange, pink, red, green, yellow and blue and thelike. The synthetic fire opal may be colorless also. Further, thesynthetic fire opal is transparent. It will be apparent to a personskilled in the art that synthetic fire opal prepared by the disclosedprocess may be of any size and can be transparent or translucent.

The synthetic fire opal prepared in accordance with an embodiment of thepresent invention has similar physical and chemical properties as anatural opal. The process for the preparation of the synthetic fire opaltakes about 4 to 6 weeks, which is substantially less, compared toalready known processes of production of synthetic opals. Further, theprocess for the preparation of the synthetic fire opal in the presentinvention is not affected by the particle size of the raw material.There is no requirement for uniform particle size of silica. The presentinvention leads to the preparation of synthetic fire opal having coloruniformity. Further, the synthetic fire opal so prepared may be of anycolor i.e., orange, pink, red, green, yellow and blue and the like. Thesynthetic fire opal may also be colorless. The synthetic fire opal ishard and transparent. The synthetic fire opal is more stable and doesnot show crazing. The synthetic fire opal can be used to make beads,cabochons cut stones or other shapes that can be used in various kindsof jewelry and ornaments.

While various embodiments of the invention have been illustrated anddescribed, it will be clear that the invention is not limited to theseembodiments only. Numerous modifications, changes, variations,substitutions, and equivalents will be apparent to those skilled in theart without departing from the spirit and scope of the invention asdescribed in the claims.

1. A process for preparing a fire opal for use in jewelry and ornaments,the process comprising the steps of: a. mixing tetraethylorthosilicate(TEOS), at room temperature and atmospheric pressure, with ethanol,concentrated nitric acid, distilled water and at least one inorganicsalt to obtain a clear sol; b. gelating the clear sol at roomtemperature days to obtain a gel; c. drying the gel at temperaturesranging from 50° C. to 100° C. to obtain a colored and transparentmaterial; and d. sintering the dried gel at temperatures ranging from250° C. to 800° C. to obtain a colored, transparent and hard fire opal.2. The process according to claim 1, wherein the inorganic salt is asalt of iron, the salt of iron being used to obtain an orange syntheticfire opal.
 3. The process according to claim 1, wherein the inorganicsalt is a salt of cobalt, the salt of cobalt being used to obtain a pinkfire opal.
 4. The process according to claim 1, wherein the inorganicsalt is a salt of cobalt, the salt of cobalt being used to obtain a redfire opal.
 5. The process according to claim 1, wherein the inorganicsalt is a salt of nickel, the salt of nickel being used to obtain agreen fire opal.
 6. The process according to claim 1, wherein theinorganic salt is a salt of copper, the salt of copper being used toobtain a blue fire opal.
 7. The process according to claim 1, whereinthe step of mixing is carried out by stirring for 1 to 10 hours toobtain a clear sol.
 8. The process according to claim 1, wherein thestep of gelating comprises storing the clear sol at room temperature ina closed cupboard.
 9. The process according to claim 1, wherein the stepof drying is performed at temperatures ranging from 50° C. to 100° C. ina laboratory furnace.
 10. The process according to claim 9, wherein thestep of drying comprises the step of maintaining the drying temperatureuntil a colored and transparent material is obtained.
 11. The processaccording to claim 1, wherein the colored and hard transparent materialobtained after drying is a silica matrix.
 12. The process according toclaim 1, wherein the step of drying comprises removing water contentfrom the pores of silica matrix.
 13. The process according to claim 1,wherein the step of sintering takes place at temperatures ranging from250° C. to 800° C.
 14. The process according to claim 13, wherein thestep of sintering comprises the step of maintaining the sinteringtemperature until a colored, transparent and hard synthetic fire opal isobtained.
 15. The process according to claim 1, wherein the step ofsintering further comprises heat treating the dried gel above 1000° C.16. The process according to claim 1, wherein the step of sinteringcomprises removing the organic part of the colored and transparentmaterial.
 17. The process according to claim 1, wherein the step ofsintering comprises reducing the water content of the colored andtransparent material.
 18. A fire opal made according to the process ofclaim
 1. 19. A process for preparing a fire opal for use in jewelry andornaments, the process comprising the steps of: a. mixingtetraethylorthosilicate (TEOS), at room temperature and atmosphericpressure, with ethanol, concentrated nitric acid and distilled water toobtain a clear sol; b. gelating the clear sol at room temperature for atime period ranging from 5 to 10 days to obtain a gel; wherein thegelating comprises the step of storing the clear sol at room temperaturein a closed cupboard; c. drying the gel at temperatures ranging from 50°C. to 100° C. in a laboratory furnace to obtain a colorless andtransparent material; wherein the drying comprises the step ofmaintaining the temperature for a few hours before cooling; and d.sintering the dried gel at temperatures ranging from 250° C. to 800° C.to obtain a colorless, transparent and hard fire opal; wherein thesintering comprises the step of maintaining the temperature for a fewhours before cooling.
 20. The process according to claim 19, wherein thestep of sintering comprises removing the organic part of the colorlessand transparent material.
 21. The process according to claim 19, whereinthe step of sintering comprises reducing the water content of thecolorless and transparent material.
 22. A synthetic fire opal for use injewelry and ornaments, the synthetic fire opal characterized by: a.hardness of 5.5 to 6.5 on Mho's scale; b. refractive index of 1.37 to1.47; c. specific gravity of 1.96 to 2.20; and d. a color from the groupconsisting of orange, pink, red, green, yellow and blue.
 23. Thesynthetic fire opal according to claim 22, wherein the fire opal issemi-transparent.
 24. The synthetic fire opal according to claim 22,wherein the synthetic fire opal contains at least two percent of watercontent.